Chelate complexes
In complex chemistry, the term chelate complexes - or chelates for short - stands for compounds in which a multidentate ligand (has more than one free electron pair) occupies at least two coordination sites (binding sites) on the central atom. In this case, the ligand is called a chelator (Greek Χηλή, chele for “crab claw ”). The central atom is usually a doubly positively charged metal ion ( e.g. Fe 2+ , Cu 2+ ). The ligands and central atom are linked by coordinative bonds . This means that the binding electron pair is provided by the ligand alone.
Chelate complexes are more stable than identical complexes with monodentate ligands that are not linked to one another. This “chelation effect” has two causes. On the one hand, the decrease in entropy during complex formation is lower, which has a thermodynamic stabilization effect. In the example above, a complex is formed from two particles. In the case of non-chelating ligands, seven particles would combine to form a complex. On the other hand, a chelating ligand can only remove itself from the central atom after all bonds have been broken, which means that the chelator is much more difficult to dissociate from the metal ion. This also increases the likelihood of immediate recombination after cleavage.
Examples of this are found in nature such as heme , chlorophyll or vitamin B12 . In medicine, it is important in the context of chelation therapy . B. EDTA (ethylenediaminetetraacetate). It is also used in analytical chemistry for titrations in chelatometry .
Chelating ligands
Important chelating ligands are:
Ligand | Abbr. | Tenacity |
---|---|---|
Acetylacetone | acac | 2 |
Ethylenediamine | en | 2 |
2- (2-aminoethylamino) ethanol | AEEA | 3 |
Diethylenetriamine | serve | 3 |
Iminodiacetate | ida | 3 |
Triethylenetetramine | trien, TETA | 4th |
Triaminotriethylamine | tren | 4th |
Nitrilotriacetate | nta | 4th |
Bis (salicylidene) ethylenediamine | salen | 4th |
Ethylenediaminotriacetate | ted | 5 |
Ethylenediaminetetraacetate | EDTA | 6th |
Diethylenetriamine pentaacetate | DTPA | 8th |
Triethylenetetramine hexaacetate | TTHA | 10 |
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate | DOTA | 8th |
Oxalate | ox | 2 |
Tartrate | tart | 2 |
Citrate | cit | 3 |
Dimethylglyoxime | dmg | 2 |
8-hydroxyquinoline | oxine | 2 |
2,2'-bipyridine | bpy | 2 |
1,10-phenanthroline | phen | 2 |
Dimercaptosuccinic acid | DMSA | 2 |
1,2-bis (diphenylphosphino) ethane | dppe | 2 |
Individual evidence
- ↑ Wolfgang Forth, Dietrich Henschler , Walter Rummel: General and special pharmacology and toxicology . First edition: Bibliographisches Institut 1975.